From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2B815C433ED for ; Mon, 26 Apr 2021 23:11:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0DC7D61027 for ; Mon, 26 Apr 2021 23:11:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234550AbhDZXMS (ORCPT ); Mon, 26 Apr 2021 19:12:18 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49984 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232022AbhDZXMR (ORCPT ); Mon, 26 Apr 2021 19:12:17 -0400 Received: from mail-yb1-xb33.google.com (mail-yb1-xb33.google.com [IPv6:2607:f8b0:4864:20::b33]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7BA07C061574; Mon, 26 Apr 2021 16:11:35 -0700 (PDT) Received: by mail-yb1-xb33.google.com with SMTP id 130so23190711ybd.10; Mon, 26 Apr 2021 16:11:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=CPYnxk6L4FHxs/fWVyMKAn+lpl/tH2wnXHBMgeYKd/A=; b=Z7yZfeG48GTsv3tCFxLE8hSu798CTwjySsCGZXBSDDO9uKg+h9G4LYGcAPwFc+XVNl +iGw7/4GjwGhp1qNIMABgpQAWwigrMrioeQVkbzqyuT9AUes3AM3n9cWwNt6YWjNimIY s7u7OXn2WgNx1bqGmjLIYH8rsBERWKRc2ntD32ka5aRMEsAToKyPN5xIWGvqxFbMUhTT zJfkupwcni1O33GMsrweMOGeTQjm/kI4irt+uOttrOs3tV/FUxEeKXtWbf8yuAv1lMmO Fl5Ie/cqkcG5xeMtT/WwVk0OqBCVWV+kzYGlViVnJyAvIt4++AuQs4eOsC7Jbua/mNsI PszA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=CPYnxk6L4FHxs/fWVyMKAn+lpl/tH2wnXHBMgeYKd/A=; b=Ja6URyf3hhyuk1IoHr4+ZfOK1kzC0cjB9q00lpJURUcrUhodAaIqE4lOPbgHp+Z66P jKUv6xr9izSS+XE4MUtUMEaxj3Zt9mkJUIAlaxiOi2+2CusLKC8/s1GmzD88Wv6kv8kO s1j9Ml/NKZi8qELh3P6JfLjigFLQIJ7d0sh8Q5LTJc6tyFCfvvSq/7l0b6BB3NOkSVqT hKgcLaFXF/Iw7ha//dvdGZzj51pLuanq+61Bz3oA9RelQRpobarWn/t+erhspmyYFfjO UNnaIa2yMayp+ObCGK7nr5rCfihXb+W60uUfab4r0oum6OEqVSEHhepkbJhxygEGEh3d jLzA== X-Gm-Message-State: AOAM530fBLW0VpGsFS1l3vSfKwasfxjxIhb3GjHJZy0Cz/3TwbeyfsXy u2ZHMYhJ/8hVLYujQ9IUfOWJN8kt/lwAm+pKTgFB3PgB X-Google-Smtp-Source: ABdhPJz/wVA1V9cjT+nfmrGcwlQwq/MB+CbcRlBEPDC6LTsNXIHrHVUM01PfpucxLSmW47lW2yRdfYFz+vHJsnKnw98= X-Received: by 2002:a25:2441:: with SMTP id k62mr27542430ybk.347.1619478694789; Mon, 26 Apr 2021 16:11:34 -0700 (PDT) MIME-Version: 1.0 References: <20210423185357.1992756-3-andrii@kernel.org> <2b398ad6-31be-8997-4115-851d79f2d0d2@fb.com> <065e8768-b066-185f-48f9-7ca8f15a2547@fb.com> <20210426223449.5njjmcjpu63chqbb@ast-mbp.dhcp.thefacebook.com> In-Reply-To: <20210426223449.5njjmcjpu63chqbb@ast-mbp.dhcp.thefacebook.com> From: Andrii Nakryiko Date: Mon, 26 Apr 2021 16:11:23 -0700 Message-ID: Subject: Re: [PATCH v2 bpf-next 2/6] libbpf: rename static variables during linking To: Alexei Starovoitov Cc: Yonghong Song , Andrii Nakryiko , bpf , Networking , Alexei Starovoitov , Daniel Borkmann , Kernel Team Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Mon, Apr 26, 2021 at 3:34 PM Alexei Starovoitov wrote: > > On Mon, Apr 26, 2021 at 08:44:04AM -0700, Andrii Nakryiko wrote: > > > > > > > > > Static maps are slightly different, because we use SEC() which marks > > > > them as used, so they should always be present. > > > > > > yes. The used attribute makes the compiler keep the data, > > > but it can still inline it and lose the reference in the .text. > > > > At least if the map is actually used with helpers (e.g., > > bpf_map_lookup_elem(&map, ...)) it would be invalid for compiler to do > > anything crazy with that map reference, because compiler has no > > visibility into what opaque helpers do with that memory. So I don't > > think it can alias multiple maps, for instance. So I think static maps > > should be fine. > > Yeah. That makes sense. > > > See above about passing a pointer to map into black box functions. I'd > > bet that the compiler can't merge together two different references at > > least because of that. > > > > For static maps, btw, just like for static functions and vars, there > > is no symbol, it's an offset into .maps section. We use that offset to > > identify the map itself. > > Ok. Sounds like there is a desire to expose both static and static volatile > into skeleton. > Sure, but let's make it such the linking step doesn't change the skeleton. > Imagine a project that using single .bpf.c file and skeleton. > It grows and wants to split itself into multiple .bpf.c. > If such split would change the skeleton generated var/map names > it would be annoying user experience. It's surely not ideal, but it's a one-time step and only when user is ready to switch to linker, so I don't see it as such a big problem. > > I see few options to avoid that: > - keeping the btf names as-is during linking > The final .o can have multiple vars and maps with the same name. > The skeleton gen can see the name collision and disambiguate them. > Here I think it's important to give users a choice. Blindly appending > file name is not ideal. > How to express it cleanly in .bpf.c? I don't know. SEC() would be a bit > ugly. May be similar to core flavors? ___1 and ___2 ? Also not ideal. ___1 vs ___2 doesn't tell you which file you are accessing static variable from, you need to go and figure out the order of linking. If you look at bpf_linker__add_file() API, it has opts->object_name which allows you to specify what should be used as __. Sane default seems to be the object name derived from filename, but it's possible to override this. To allow end-users customize we can extend bpftool to allow users to specify this. One way I was thinking would be something like bpftool gen object my_obj1.o=my_prefix1 my_obj2.o=my_prefix2 If user doesn't want prefixing (e.g., when linking multi-file BPF library into a single .o) they would be able to disable this as: bpftool gen object lib_file1.o= lib_file2.o= and so on > - another option is to fail skeleton gen if names conflict. > This way the users wold be able to link just fine and traditonal C style > linker behavior will be preserved, but if the user wants a skeleton > then the static map names across .bpf.c files shouldn't conflict. > imo that's reasonable restriction. There are two reasons to use static: 1. hide it from BPF code in other files (compilation units) 2. allow name conflicts (i.e., not care about anyone else accidentally defining static variable with the same name) I think both are important and I wouldn't want to give up #2. It basically says: "no other file should interfere with my state neither through naming or hijacking my state". Obviously it's impossible to guard from user-space interference due to how BPF maps/progs are visible to user-space, so those guarantees are mostly about BPF code side. Name prefixing only affects BPF skeleton generation and user-space use of those static variables, both of which are highly-specific use patterns "bridging two worlds", BPF and user-space. So I think it's totally reasonable to specify that such variables will have naming prefixes. Especially that BPF static variables inside functions already use similar naming conventions and are similarly exposed in BPF skeleton. > - maybe adopt __hidden for vars and maps? Only not hidden (which is default now) > would be seen in skeleton? This is similar to the above, it gives up the ability to not care about naming so much, because everything is forced to be global.